This invention relates generally to the production of sheets of balsa wood usable in the construction of models and in other applications requiring a veneer of balsa wood, and in particular to a high-yield-low-cost technique for converting balsa logs into such sheets.
In making model airplanes, the preferred material is balsa wood; for this wood has unique structural properties ideally suited for this application. On the average, it weighs less than 9 pounds per cubic foot, this being 40% less than the lightest species of wood in North America. Its cell structure affords a combination of high rigidity, compressive and tensile strength superior to any composite or synthetic material of equal or higher density.
Balsa wood sheets are not only popularly used in the making model airplanes but are often employed in constructing model cars and boats as well as in the fabrication of architectural models for one can readily cut and profile balsa sheets into pieces of any desired shape and then glue or otherwise join the pieces together. Balsa wood sheets are also used as feedstock for multi-ply, light-weight plywood.
The present practice in producing balsa wood sheets usable for model-making or plywood is to cut boards of high grade balsa wood into thin, veneer-like sheets, the standard stock size being a sheet one-sixteenth of an inch thick, three inches in width and a yard in length. When, as is sometimes the case, one needs broader sheets, then three-inch wide pieces have to be joined together; for the maximum produceable width of these pieces under existing conversion techniques is limited by the relatively small diameter of balsa wood trees when they attain cutting maturity.
Balsa wood sheets, as presently produced, are relatively expensive, this being due in large part to the low yield obtained from balsa logs using conventional conversion techniques in which the logs must first be reduced to boards which are then cut into sheets. With such techniques, the amount of balsa convertible into usable board is usually less then half the total volume of wood in the log. This is due to the constraint that only rectangular or square pieces can be cut from a cylindrical log to produce a board that has a rectangular form. To this end, a series of longitudinal cuts are made through the log to produce so-called "flat-sawn" pieces whose broad faces lie in a plane parallel to a tangent to the cylindrical periphery of the log.
Flat pieces not only give rise to substantial amounts of wood waste, but such pieces tend to warp during the kiln-drying process. An even when adequately dried, flat sawn pieces undergo dimension changes as a result of variations in air moisture or relative humidity, so that the final product may become deformed.
The typical board thickness required by the trade is three inches. In producing boards by the traditional conversion technique, it is difficult to produce pieces of the required thickness that are also wide. Since balsa wood trees do not attain a diameter much greater than one foot at cutting maturity, it is obvious that in converting logs of one foot diameter into boards of three inches in thickness by the traditional technique, that these pieces will have a width averaging about four inches--especially when taking into account the waste encountered in kiln-drying as well as normal wood defects such as knots and grain distortion.
Thus in calculating the yield of balsa sheets derived from balsa logs, one must take into account not only the substantial losses resulting from the conversion of cylindrical logs into boards by the conventional flat-sawn conversion technique, but the further losses arising from the fact that only a portion of the resulting boards are of a quality acceptable for conversion into thin sheets.
It has been found that the actual yield of balsa sheets derived from logs using existing techniques runs as low as 8 percent. While balsa trees are fast-growing and reach cutting maturity in six to eight years, the plantations are almost all in South America. Rising transportation, labor and processing costs, coupled with the exceptionally low yield obtained from conventional conversion techniques are such as to make balsa wood in sheet form increasingly expensive.
Another drawback of conventionally-produced veneer-like sheets of balsa wood is that its flatness is subject to changing conditions of temperature and relative humidity in ambient air. This wood is always undergoing changes in moisture content, and when the wood is a thin sheet of balsa, long-term or seasonal changes may result in variations in moisture content causing warpage of the sheets.
Of particular interest is the prior patent to Newmark et al., U.S. Pat. No. 2,409,785, in which square bolts of balsa derived from a log are cut in a slicing machine into thin sheets suitable for model-making. While the technique disclosed in this patent overcomes some of the drawbacks of the prior art, it lacks the advantages of the present invention.